Non-porous hydrogen diffusion fuel cell electrodes



United States Patent 3,337,368 NON-POROUS HYDROGEN DIFFUSION FUEL CELLELECTRODES Harry G. Oswin, Elmsford, N.Y., assignor to LeesonaCorporation, Cranston, R.I., a corporation of Massachusetts No Drawing.Filed Aug. 24, 1960, Ser. No. 51,496 2 Claims. (Cl. 1136-86) Thisinvention relates to improved fuel cell electrodes. More particularlythe invention relates to fuel cell electrodes comprising a non-porouspalladium hydrogen-diffusion electrode.

Fuel cell, as used in this specification, is the common name applied toan electrochemical cell capable of generating electrical energy throughelectrochemical combustion of a fuel gas with an oxygen containing gas.These cells have been fully described in the literature. Their preciseconstruction and operation does not form a part of the instant inventionexcept in an incidental capacity. However, a brief description of thenature and construction of a simple fuel cell is believed helpful, ifnot essential, in understanding the function and importance of thepresent invention.

In general, the. simplest fuel cell comprises a housing, two electrodesand an electrolyte which acts as an oxygen transferring medium. Anoxidizing gas such as air under superatmosphere pressure is circulatedon one side of the oxidizing electrode and a fuel gas such as hydrogen,under super-atmospheric pressure is circulated on one side of the otherelectrode. A three-phase interface exists at each electrode, i.e., gas,electrolyte, and solid Where a process of adsorption and de-adsorptionoccurs generating an electrochemical force. When current is drained fromthe two electrodes there is a net flow of electrons from the fuel gasside through an external electrical circuit to the oxidizing gas side.Thus, according to the external electron flow convention, the oxidizinggas electrode is the positive electrode and the fuel gas electrode isthe negative electrode. Oxygen is consumed at the positive electrodesurface and fuel gas is oxidized into products of combustion at thenegative electrode surface. The result is accompanied by release of aportion of the energy of combustion as electrical energy while theremainder is released as heat.

In the past it was necessary to regulate the three-phase interface ofsolid-gas-electrolyte by a suitable combination of pore size, pressuredifferential of the gas, and surface tension of the electrolyte. As apractical matter, however, it was impossible to maintain completelyuniform pore size; thus, the cell was always operated with some of thesmaller cells flooded with electrolyte due to capillary action or withgas bubbling through the larger pores unused. To a large extent theadvent of a biporous electrode structure solved this problem. In abi-porous system, large pores front the gas of the fuel cell system andthe smaller pores face the electrolyte. A three-phase interface occurssubstantially at the bi-porous wall.

Bi porous electrodes, however, were not the complete answer to theproblem inasmuch as bi-porous structures are fabricated from carefullyfractionated metal powders having well defined pore size by a process ofsintering, compacting, etc., which results in a very expensiveelectrode. In addition, the oxidation of hydrogen at the threephaseinterface results in water formation within the porous structure whichpresents a serious removal problem. Further, the prior art electrodesrequired the use of pure hydrogen, since impurities in the gas preventeddiffusion of the hydrogen to the three-phase interface.

Accordingly, it is an object of the present invention to provide anon-porous hydrogen diffusion electrode, thus eliminating the problem offlooding and bubbling of gas through the pores.

It is another object of the invention to provide a hydrogen dilfusionelectrode capable of utilizing impure hydrogen.

It is another object of the invention to provide a hydrogen diffusionelectrode in which it is not essential to accurately control thepressure of the hydrogen fuel gas.

It is still another object of the invention to provide a hydrogendiffusion electrode which eliminates the problem of water formationwithin the porous structure.

These and other objects of the invention will be apparent from thefollowing description with particular emphasis being directed to thespecific examples.

Briefly, the objects of the instant invention are accomplished byfabricating a hydrogen diffusion electrode comprising a thin non-porouspalladium membrane through which hydrogen diffuses as protons orhydrogen atoms. In the fuel cell, the fuel gas is circulated on one sideof the membrane and the other face of the electrode fronts theelectrolyte into which the hydrogen can diffuse as a proton. The systemcan be illustrated graphically as follows, with FIG. 1 utilizing an acidelectrolyte and FIG. 2 utilizing an alkaline electrolyte.

As is apparent from the above figures hydrogen gas is diffused throughthe palladium membrane separating an electron from the hydrogen andpassing the proton into the electrolyte. The electron is drawn off andcarried, via an external route, to the oxidizing electrode by which theyare consumed.

Since only hydrogen is diffused through the palladium membrane impurehydrogen gas, containing carbon dioxide, carbon monoxide, water,methane, ammonia, etc., can be used as the fuel gas. The hydrogen willdiffuse through the membrane and the gaseous impurities can easily beremoved by suitable venting. The impurities, being concentrated insidethe membrane, cannot contaminate the electrolyte. Thus, an electrodecapable of using relatively cheap impure hydrogen is an importantfeature of the instant invention.

The instant hydrogen-diffusion electrodes can be utilized in fuel cellsystems operating in a Wide temperature range. However, for goodhydrogen diffusion it is desirable that the temperature of the system bein excess of C. but not over 700 C., with the preferred range being inthe neighborhood of ISO-300 C. While fuel cell systems comprising theinstant electrodes can be operated at lower temperatures, their behaviorat such temperatures is somewhat erratic.

The thickness of the palladium membranes for use as the electrodedepends to a large degree upon the pressure differential to be appliedacross the membrane and upon the rapidity of diffusion desired.Diffusion of hydrogen gas through the membrane is proportional to thepressure differential across the membrane and the membranes thickness.The minimum thickness is immaterial as long as the membrane isstructurally able to withstand the necessary pressure of the fuel cell.The preferred range of thickness is from approximately .05 mil to 30mils. The

membranes can be fabricated as flat supported sheets, or as a corrugatedor tubular construction. Usually tubular construction is preferred sincethe effective surface area of the electrode is increased and it is idealfor bipolar or multi-polar cells. Additionally a tubular structure willwithstand greater pressure. For example a tube of 0.003 inch thickness,having a inch outside diameter will withstand at least 1000 psi.pressure and will sustain very high current densities.

The instant electrodes can be operated with a variety of acid andalkaline electrolytes such as sulfuric acid, phosphoric acids, potassiumhydroxide, sodium hydroxide, etc. An outstanding feature of theelectrode is that the formation of water occurs only in the electrolyteand not in the electrode structure. Thus, the water does not effect thehydrogen diffusion and can be conveniently removed from the electrolyteby suitable means.

Another, and probably the most unusual and surprising feature of theinstant invention is the ability of the Pd membrane electrodes to act astheir own metering valve. It would logically be expected that anelectrode at 250 C. would bubble hydrogen under open circuit conditions.However, this is not the case with the instant systems. When the circuitis open the hydrogen does not diffuse through the membrane, but as soonas the circuit is closed the electrode responses and hydrogen gas ismetered through. This is a particularly desirable and unexpectedcharacteristic of the instant system.

The explanation for this unusual phenomenon is not understood, however,it is theorized that the hydrogen dissociates into protons and electronsat the first surface of the palladium membrane. When the protons andelectrons reach the second surface of the membrane they recombine onadjacent Pd atoms of the lattice if no electrolyte is present, reforminghydrogen gas. However, when electrolyte is present, due to the presenceof other chemisorbed ionic forms such as -OH, Na K the recombinationdoes not occur inasmuch as the surface diffusion is restricted andconsequently there are fewer PdPd and HH pairs available, needed for thediffusion. However, when the circuit is closed and electrons are drawnoff by an external route, the hydrogen protons will pass through andcombine with the hydroxyl ions of the electrolyte. The instantexplanation is only theoretical and is not intended to limit theinvention. There is no explanation from the prior art which would leadone to expect a phenomenon of this type. Thus, the electrodes of theinstant invention are distinctly superior to what would be predicted orexpected.

The following examples are set forth to more particularly illustrate theinvention. However, they are not to be construed as limiting. Otherembodiments can be conveniently produced without departing from thescope of the invention.

Example 1 A fuel cell system having a metallic Pt oxidizing electrode, apure palladium membrane of .020 inch thickness as the fuel electrode andusing an aqueous 50% H 80 electrolyte was fashioned in a suitablehousing. The cell,

operated at psi. differential pressure at 250 C., sustained a currentdensity of 200 amps/ft? In the above cell an impure gas containing 90%hydrogen and 10% nitrogen was substituted for pure hydrogen. The gas wascirculated through a tubular palladium electrode, allowing the hydrogento diffuse through the membrane and the impurities removed by venting.The cell sustained substantially the same current density, within thelimits of experimental error, as a cell using pure hydrogen fuel underthe same conditions.

Fuel cells utilizing the electrodes of the instant invention respondedvery rapidly to operating conditions and were substantially superior tonickel electrodes under similar conditions. However, it was noted thatthe operating efliciency of fuel cells utilizing the instant hydrogendifiusion electrodes were slightly impaired by'substantial amounts ofolefinic compounds in contact with the electrode because of electrodepoisoning. This feature can be easily remedied by reactivating theelectrode by flushing the membrane with oxygen gas at temperatures offrom about 200-500 C. Additionally, it may be desirable to activate themembrane by surface treatment at either the gas or electrolyte face witha very thin film of another metal such as nickel or platinum to maintainhigh halfcell potentials.

The instant invention is not to be limited by the illustrated example.It is possible to produce still other embodiments without departing fromthe inventive concept herein disclosed. Such embodiments are within theability of one skilled in the art.

It is claimed and desired to be secured by letters Patent:

1. A fuel cell for the direct generation of electricity comprising anon-porous hydrogen diffusion palladium membrane anode, a porous cathodeand an aqueous electrolyte.

2. In a fuel cell comprising a fuel electrode, at least one porousoxidizing electrode and an electrolyte, the improvement wherein hydrogenis employed as the fuel and the fuel electrode is a non-porous palladiummembrane and said hydrogen fuel diffuses through said membrane.

References Cited UNITED STATES PATENTS 353,141 11/1886 Kendall 136-862,749,293 6/1956 Wahlin 204-73 2,901,523 8/1959 Justi et al 136-862,913,511 11/1959 Grubb 136-86 2,927,888 3/1960 Beard 204- FOREIGNPATENTS 521,773 5/1940 Great Britain.

WINSTON A. DOUGLAS, Primary Examiner.

JOHN R. SPECK, JOSEPH REBOLD, J-OHN R. MACK,

Examiners.

S. H. PARKER, J. E. CARSON, H. FEELEY,

Assistant Examiners.

1. A FUEL CELL FOR THE DIRECT GENERATION OF ELECTRICITY COMPRISING ANON-POROUS HYDROGEN DIFFUSION PALLADIUM MEMBRANE ANODE, A POROUS CATHODEAND AN AQUEOUS ELECTROLYTE.